Inhibition of calcium-dependent nitric oxide synthase causes ileitis and leukocytosis in guinea pigs

Induction of Nitric-Oxide Metabolism in Enterocytes Alleviates Colitis and Inflammation-Associated Colon Cancer

Nitric oxide (NO) plays an established role in numerous physiological and pathological processes, but the specific cellular sources of NO in disease pathogenesis remain unclear, preventing the implementation of NO-related therapy. Argininosuccinate lyase (ASL) is the only enzyme able to produce arginine, the substrate for NO generation by nitric oxide synthase (NOS) isoforms. Here, we generated cell-specific conditional ASL knockout mice in combination with genetic and chemical colitis models. We demonstrate that NO derived from enterocytes alleviates colitis by decreasing macrophage infiltration and tissue damage, whereas immune cell-derived NO is associated with macrophage activation, resulting in increased severity of inflammation. We find that induction of endogenous NO production by enterocytes with supplements that upregulate ASL expression and complement its substrates results in improved epithelial integrity and alleviation of colitis and of inflammation-associated colon cancer.

Rotenone and elevated extracellular potassium concentration induce cell-specific fibrillation of α-synuclein in axons of cholinergic enteric neurons in the guinea-pig ileum

BACKGROUND

Parkinson's disease is a progressive neurodegenerative disorder that results in the widespread loss of select classes of neurons throughout the nervous system. The pathological hallmarks of Parkinson's disease are Lewy bodies and neurites, of which α-synuclein fibrils are the major component. α-Synuclein aggregation has been reported in the gut of Parkinson's disease patients, even up to a decade before motor symptoms, and similar observations have been made in animal models of disease. However, unlike the central nervous system, the nature of α-synuclein species that form these aggregates and the classes of neurons affected in the gut are unclear. We have previously reported selective expression of α-synuclein in cholinergic neurons in the gut (J Comp Neurol. 2013; 521:657), suggesting they may be particularly vulnerable to degeneration in Parkinson's disease.

METHODS

In this study, we used immunohistochemistry to detect α-synuclein oligomers and fibrils via conformation-specific antibodies after rotenone treatment or prolonged exposure to high [K⁺ ] in ex vivo segments of guinea-pig ileum maintained in organotypic culture.

KEY RESULTS

Rotenone and prolonged raising of [K⁺ ] caused accumulation of α-synuclein fibrils in the axons of cholinergic enteric neurons. This took place in a time- and, in the case of rotenone, concentration-dependent manner. Rotenone also caused selective necrosis, indicated by increased cellular autofluorescence, of cholinergic enteric neurons, labeled by ChAT-immunoreactivity, also in a concentration-dependent manner.

CONCLUSIONS & INFERENCES

To our knowledge, this is the first report of rotenone causing selective loss of a neurochemical class in the enteric nervous system. Cholinergic enteric neurons may be particularly susceptible to Lewy pathology and degeneration in Parkinson's disease.

Nitric oxide as a mediator of inflammation?-You had better believe it

Nitric oxide has enigmatic qualities in inflammation. In order to appreciate the precise contributions of nitric oxide to a pathophysiological process, one must account for enzyme source, coproduction of oxidants and antioxidant defences, time, rate of nitric oxide production, cellular source, peroxynitrite formation and effects on DNA (mutagenesis/apoptosis). We contend that there is ample evidence to consider nitric oxide as a molecular aggressor in inflammation, particularly chronic inflammation. Therapeutic benefit can be achieved by inhibition of inducible nitric oxide synthase and not the donation of additional nitric oxide. Furthermore, there is growing appreciation that nitric oxide and products derived thereof, are critical components linking the increased incidence of cancer in states of chronic inflammation.

Knock out of neuronal nitric oxide synthase exacerbates intestinal ischemia/reperfusion injury in mice

Recent investigation of the intestine following ischemia and reperfusion (I/R) has revealed that nitric oxide synthase (NOS) neurons are more strongly affected than other neuron types. This implies that NO originating from NOS neurons contributes to neuronal damage. However, there is also evidence of the neuroprotective effects of NO. In this study, we compared the effects of I/R on the intestines of neuronal NOS knockout (nNOS(-/-)) mice and wild-type mice. I/R caused histological damage to the mucosa and muscle and infiltration of neutrophils into the external muscle layers. Damage to the mucosa and muscle was more severe and greater infiltration by neutrophils occurred in the first 24 h in nNOS(-/-) mice. Immunohistochemistry for the contractile protein, α-smooth muscle actin, was used to evaluate muscle damage. Smooth muscle actin occurred in the majority of smooth muscle cells in the external musculature of normal mice but was absent from most cells and was reduced in the cytoplasm of other cells following I/R. The loss was greater in nNOS(-/-) mice. Basal contractile activity of the longitudinal muscle and contractile responses to nerve stimulation or a muscarinic agonist were reduced in regions subjected to I/R and the effects were greater in nNOS(-/-) mice. Reductions in responsiveness also occurred in regions of operated mice not subjected to I/R. This is attributed to post-operative ileus that is not significantly affected by knockout of nNOS. The results indicate that deleterious effects are greater in regions subjected to I/R in mice lacking nNOS compared with normal mice, implying that NO produced by nNOS has protective effects that outweigh any damaging effect of this free radical produced by enteric neurons.

Arginine does not exacerbate markers of inflammation in cocultures of human enterocytes and leukocytes

Enteral arginine supplementation in the critically ill has become a matter of controversy. In this study, we investigated effects of the addition of 0.4 and 1.2 mmol/L arginine in a coculture model on markers of inflammation, enterocyte layer integrity, and amino acid transport. In this model, a monolayer of intestinal epithelial cells (Caco-2) separated compartments with nonpathogenic Escherichia coli and mononuclear leukocytes. Activation of enterocytes and leukocytes was assessed by the measurement of nitric oxide, TNF-alpha, IL-6, IL-8, IL-10, and IFN-gamma. Further outcomes were the transepithelial flux of 22 amino acids, their catabolism, and the integrity of the enterocyte layer assessed as permeability of fluorescein dextran (M(r) 4400). Bacterial stimulation of intestinal epithelial cells enhanced the basolateral concentration of nitric oxide and all cytokines measured. Supplementation with arginine did not affect epithelial integrity, production of any of the cytokines investigated, or the amount of nitric oxide. The amino acid used primarily by nonstimulated intestinal epithelial cells cocultured with leukocytes was glutamine. Activation of IEC with bacteria significantly enhanced the catabolism of serine, asparagine, and lysine, and reduced glutamine catabolism. Addition of arginine increased ornithine formation and moderately reduced transepithelial transport of methionine and other amino acids. Hence, arginine supplementation does not interfere with inflammation-associated cross-talk between human enterocytes and leukocytes. Because it also does not seem to affect the integrity of enterocyte layers, a detrimental role of arginine during septic-like conditions seems unlikely.

Protective effect of dietary nitrate on experimental gastritis in rats

Nitrates have long been considered as harmful dietary components and judged responsible for deleterious effects on human health, leading to stringent regulations concerning their levels in food and water. However, recent studies demonstrate that dietary nitrate may have a major role in human health as a non-immune mechanism for host defence, through its metabolism to NO in the stomach. NO is a versatile molecule and although evidence exists showing that administration of low doses of exogenous NO protects against gastrointestinal inflammation, higher NO doses have been shown to exacerbate injury. So, the effect of an ingestion of nitrates in doses corresponding to a normal diet in human consumers on an experimental gastritis induced by iodoacetamide in rats was investigated. During gastritis one of the following compounds was given orally: water; KNO3; the NO donor sodium nitroprusside; the NO scavenger haemoglobin given with either water or KNO3. N(G)-nitro-l-arginine methyl ester (l-NAME), a non-specific NO synthase inhibitor, was administered with either water, iodoacetamide alone, or combined with KNO3. After killing, the stomach was resected and microscopic damage scores, myeloperoxidase and NO synthase activities were determined. Iodoacetamide-induced gastritis was significantly reduced by KNO3 administration, an effect which was reproduced by sodium nitroprusside and reversed by haemoglobin. l-NAME induced gastric mucosal damage in itself, and KNO3 did not prevent the gastritis induced by iodoacetamide associated with l-NAME. In conclusion, dietary nitrate exerts a protective effect against an experimental gastritis in rats by releasing NO in the stomach but such an effect requires the production of endogenous NO.

Role of nitric oxide in hemorrhagic shock-induced bacterial translocation

BACKGROUND

Hemorrhagic shock-induced bacterial translocation is an etiologic factor in the pathogenesis of multiple system organ damage. Excessive production of nitric oxide (NO) during hemorrhagic shock may lead to cellular injury and gut barrier failure that promotes bacterial translocation. We investigated the effect of aminoguanidine (AG) and N(G)-nitro-l-arginine methyl ester (l-NAME), both inhibitors of NO synthase, on hemorrhagic shock- induced bacterial translocation in the rat.

MATERIALS AND METHODS

Anesthetized male Sprague-Dawley rats were subjected to a hemorrhagic shock protocol for 30 min followed by intravenous injection (1 mL/kg body wt) with normal saline, AG (100 mg/kg), or l-NAME (10 mg/kg). Tissues/organs were examined histologically for damage and bacterial translocation. Plasma nitrate/nitrite was measured using a procedure based on the Griess reaction, and nitric oxide synthase (NOS) expression was determined immunohistochemically.

RESULTS

The shocked animals treated with saline died within 90 min, and deaths were associated with 100% bacterial translocation, increased tissue/organ damage, and elevated nitrate/nitrite production. In contrast, both AG and l-NAME increased the survival time of shocked rats to >72 h, abrogated bacterial translocation, reduced tissue/organ damage, and prevented excessive nitrate/nitrite production and upregulation of expression of endothelial NOS and inducible NOS.

CONCLUSIONS

Prevention of bacterial translocation by pharmacologic agents such as aminoguanidine and l-NAME could be an important therapeutic approach to lessen mortality rates following hemorrhagic shock.

Nitric oxide and intestinal inflammation

Inflammation of the intestinal tract remains a very serious concern in the clinical setting. Unfortunately, to date, the mechanisms underlying many inflammatory conditions such as sepsis or inflammatory bowel diseases are poorly understood and our therapeutic interventions are less than ideal. Over the past decade, an abundance of research has been directed toward the role of nitric oxide (NO) in intestinal inflammation. It has become apparent that NO might have a dichotomous role as both a beneficial and detrimental molecule. Nitric oxide is a weak radical produced from L-arginine via the enzyme nitric oxide synthase (NOS). NOS exists in three distinct isoforms; constitutively (cNOS) expressed neuronal NOS (NOS1 or nNOS) and endothelial NOS (NOS3 or eNOS) or an inducible isoform (NOS2 or iNOS) capable of high production output of NO during inflammation. Constitutively expressed NOS has been shown to be critical to normal physiology and inhibition of these enzymes (nNOS or eNOS) caused damage. It has been proposed that the high output production of NO from iNOS causes injury, perhaps through the generation of potent radicals such as peroxynitrite and hence may explain the apparent dichotomous role of NO. However, recent studies have challenged this simple paradigm providing evidence that iNOS may have some protective role in some inflammatory models. Moreover, the importance of peroxynitrite has been questioned. In this review we discuss the role of cNOS and iNOS in intestinal inflammation and provide an overview of peroxynitrite in intestinal inflammation, highlighting some of the controversy that exists.

Novel anti-inflammatory chalcone derivatives inhibit the induction of nitric oxide synthase and cyclooxygenase-2 in mouse peritoneal macrophages

In a previous work, we tested a series of chalcone derivatives as possible anti-inflammatory compounds. We now investigate the effects of three of those compounds, CHI, CH8 and CH12, on nitric oxide and prostanoid generation in mouse peritoneal macrophages stimulated with lipopolysaccharide and in the mouse air pouch injected with zymosan, where they showed a dose-dependent inhibition with inhibitory concentration 50% values in the microM range. This effect was not the consequence of a direct inhibitory action on enzyme activities. Our results demonstrated that chalcone derivatives inhibited de novo inducible nitric oxide synthase and cyclooxygenase-2 synthesis, being a novel therapeutic approach for inflammatory diseases.

Roles of nitric oxide synthases in platelet-activating factor-induced intestinal necrosis in rats

OBJECTIVE

To examine the role of constitutive and inducible nitric oxide synthases (cNOS and iNOS) in platelet-activating factor (PAF)-induced shock and intestinal injury.

DESIGN

Prospective, randomized, controlled experimental study.

SETTING

Hospital research laboratory.

SUBJECTS

Young adult male Sprague-Dawley rats were anesthetized and studied.

INTERVENTIONS

Rats were injected with PAF, either alone or after the following pretreatments: a) selective iNOS inhibitors aminoguanidine or S-methylisothiourea; b) 3-morpholinosydnonimine, a NO donor; c) S-methylisothiourea + 3-morpholinosydnonimine; and d) antineutrophil antibody (to deplete neutrophils).

MEASUREMENTS AND MAIN RESULTS

Blood pressure, hematocrit, white blood cell counts, intestinal injury, and intestinal cNOS and iNOS activities were assessed. We found that: a) cNOS is the predominant NOS in the intestine and its activity is inversely correlated to the level of tissue injury; b) there is a time-dependent increase in cNOS activity in sham-operated animals, which was abolished by PAF; c) Western blotting and immunohistochemistry showed iNOS present in the normal intestine, localizing mainly in crypt cells; d) iNOS inhibitors attenuated PAF-induced injury in animals with high cNOS activity, but had no protective effect in animals with low cNOS activity; e) 3-morpholinosydnonimine, alone or together with S-methylisothiourea, alleviated PAF-induced injury; and f) neutrophil depletion blocked the suppressive effect of PAF on cNOS and prevented injury.

CONCLUSIONS

We conclude that cNOS and iNOS play different roles in PAF-induced intestinal injury. Caution should be exerted concerning potential therapeutic uses of iNOS inhibitors.

The relationships among nitric oxide production, bacterial translocation, and intestinal injury after endotoxin challenge in vivo

BACKGROUND

This study examines the hypothesis that there is a relationship among endotoxin-induced bacterial translocation (BT) and increased nitric oxide (NO) production and that inhibition of excessive NO production with NG-monomethyl-L-arginine (L-NMMA) is beneficial.

METHODS

Rats received 0, 1, or 4 mg/kg endotoxin intraperitoneally, and 6 or 18 hours later, they were killed and BT, NO production (as reflected in nitrite/nitrate levels), and calcium-dependent nitric oxide synthase and calcium-independent nitric oxide synthase (iNOS) activities were measured in tissues (ileum, liver, and mesenteric lymph nodes) and blood. In a second set of experiments, the animals received the NOS inhibitor L-NMMA (100 mg/kg) intravenously either 15 minutes before or 2 hours after endotoxin challenge (4 mg/kg) and the same parameters were measured.

RESULTS

The incidence of BT was higher in rats receiving 4 mg/kg endotoxin (62.5%) than in the control group (0%, p < 0.05), and the 1 mg/kg endotoxin group had intermediate incidence (25%). The animals receiving 4 mg/kg endotoxin had higher tissue (mesenteric lymph nodes, liver) and blood nitrite/nitrate levels than the control or 1 mg/kg endotoxin groups. The increased NO production was mainly attributable to an elevated level of iNOS activity. The administration of L-NMMA before but not after endotoxin challenge reduced iNOS activity, NO production, and BT to control levels at 6 hours but not 18 hours after endotoxin administration.

CONCLUSION

Endotoxin-induced mucosal injury and BT are associated with iNOS activity and increased NO production. Inhibition of iNOS activity with L-NMMA before treatment prevented endotoxin-induced ileal mucosal injury and BT.

Failure of L-NAME to cause inhibition of nitric oxide synthesis: role of inducible nitric oxide synthase

We addressed the hypothesis that administration of nitric oxide synthase inhibitor, NG -nitro-L-arginine methyl ester (L-NAME) does not result in a sustained suppression of nitric oxide (NO) synthesis, because of a compensatory expression of inducible nitric oxide synthase (iNOS). L-NAME was administered in the drinking water (0.1-1.0 mg/ml) for 7 days to guinea pigs and rats. Nitric oxide synthesis was assessed by [1] ex vivo formation of nitrite in blood vessels and intestine [2] tissue levels of cGMP [3] iNOS gene expression by RT-PCR [4] NADPH diaphorase staining [5] direct assessment of NO release in tissue explants using a microelectrode/electrochemical detection system. Chronic L-NAME administration elevated intestinal cGMP and nitrite levels in guinea pigs (p < 0.05). In rats, intestinal nitrite levels were comparable in control and L-NAME treatment groups, whereas direct assessment of NO release defined a marked increase in the L-NAME group. Chronic L-NAME resulted in an induction of iNOS gene expression in rats and guinea pigs and novel sites of NADPH diaphorase staining in the intestine. We conclude that iNOS expression is responsible for a compensatory increase or normalization of NO synthesis during sustained administration of L-NAME.

Relationship between nitric oxide and platelet-activating factor in castor-oil induced mucosal injury in the rat duodenum

The modulation of platelet activating factor (PAF) formation in duodenal tissue by nitric oxide (NO) released in response to castor oil was studied in rats pretreated with NG-nitro-L-arginine methyl ester (L-NAME, 6.25-25 mg/kg, i.p.), an inhibitor of NO synthase, NG-nitro-D-arginine methyl ester (D-NAME, 25 mg/kg, i.p.), the inactive enantiomer of L-NAME or isosorbide-5-mononitrate (IMN, 30-90 mg/kg, p.o.), a NO donating compound. Castor oil (2 ml/rat orally) increased PAF production in the rat duodenum 3 h after challenge. L-NAME, but not D-NAME, enhanced the amount of PAF formed by duodenal tissue, while IMN (30-90 mg/kg) counteracted the effects of L-NAME (12.5 mg/kg) and also reduced PAF release in the tissue of rats treated with castor oil. L-NAME 12.5 mg/kg, but not D-NAME, enhanced both macroscopic damage and acid phosphatase release induced by castor oil. These effects were reduced by a PAF antagonist BN 52021 (3-t-Butyl-hexahydro-4, 7b, 11-trihydroxy-8-methyl-9H-1, 7a-epoxymethano-1H, 6aH-cyclopenta [c] furo [2, 3b] furo [3'2':3,4] cyclopenta [1.2-d]furan-5,9,12(4H)trione) 10 and 20 mg/kg i.p. Such findings suggest that endogenous nitric oxide could reduce PAF biosynthesis in castor oil-treated rats.

Fetal growth retardation in rats may result from apoptosis: role of peroxynitrite

Administration of the nitric oxide synthase (NOS) inhibitor NG-nitro-L-arginine methyl ester (L-NAME) results in fetal growth retardation. This study was designed to further examine the influence of NO on fetal growth, specifically, the potential role of inducible NOS and to evaluate the possibility that apoptosis contributed to uteroplacental dysfunction. L-NAME administration caused a paradoxical increase in NO synthesis determined by direct detection of NO by electrochemistry, nitrite accumulation, and cGMP levels, indicating that a lack of NO was not the cause of the fetal growth retardation. Additionally, supplemental L-arginine or NO donors failed to reverse the effects of L-NAME on fetal and placental size. Administration of low dose endotoxin (30 micrograms/kg IP daily for 6 d) also caused significant reductions in fetal and placental size and increased NO synthesis comparable to that seen with L-NAME. Inducible NOS was constitutively expressed in the pregnant uterus (smooth muscle and epithelia) and placenta (sinusoids and macrophages) but was absent in the nonpregnant state as determined by RT-PCR and immunohistochemistry. Neither L-NAME nor endotoxin modified the expression of iNOS. In situ evidence for apoptosis (DNA fragmentation) was minimal to absent in control pregnant rats, but markedly evident in the placenta (decidua) and uterus of rats treated with L-NAME or endotoxin. Immunohistochemical evidence for nitrotyrosine, a marker for peroxynitrite formation, was absent in control rats but colocalized with apoptosis in the L-NAME and LPS groups. We conclude that L-NAME-induced fetal growth retardation is not due to a lack of NO, but as for endotoxin, results from a net reduction in cellular proliferation due to the induction of apoptosis, possibly in response to peroxynitrite formation.

The effects of aminosalicylic acid derivatives on nitric oxide in a cell-free system

AIMS AND METHODS

To determine the effect of aminosalicylic acid derivatives on the concentration of nitric oxide produced in a cell-free system, by the use of a sensitive and specific polarographic meter.

RESULTS

The aminosalicylic acid derivatives 3-ASA (IC50 100 microM), 4-ASA (IC50 350 microM) and 5-ASA (IC50 5 microM) all decreased the nitric oxide signal. These drugs had a similar inhibitory effect on the formation in vitro of nitrite from sodium nitroprusside (IC50: 200 microM, 500 microM and 100 microM, respectively). Sulphasalazine (31.1 +/- 5% decrease in signal at 1 mM) was less effective than 5-ASA, but sulphapyridine, N-acetyl 5-ASA, indomethacin and hydrocortisone produced no decrease in nitric oxide signal at all.

CONCLUSIONS

Nitric oxide binding may be part of the mechanism by which ASA derivatives exert their therapeutic effect, and this work suggests that it may be an important factor in the pathogenesis of ulcerative colitis.

Nitric oxide inhibition causes intrauterine growth retardation and hind-limb disruptions in rats

OBJECTIVE

Our purpose was to determine the effects of nitric oxide synthase inhibition on maternal and fetal health in the last third of pregnancy.

STUDY DESIGN

Pregnant rats were treated from gestational day 13 to day 19 or 20 with the nitric oxide synthase inhibitor NG-nitro-L-arginine methyl ester, which was administered in the drinking water ad libitum. Control animals received the inactive enantiomer NG-nitro-D-arginine methyl ester or no treatment. Maternal blood pressure, blood chemistry studies, and placenta and pup size were determined. A separate group of rats received nitroprusside sodium in conjunction with NG-nitro-L-arginine methyl ester.

RESULTS

NG-nitro-L-arginine methyl ester caused a dose-dependent reduction in placenta and pup size. Amniotic fluid levels of cyclic guanosine monophosphate were significantly reduced at 0.1 mg/ml but not at higher doses. Hemorrhagic necrosis of fetal hind limbs occurred only with treatment with NG-nitro-L-arginine methyl ester and was prevented by coadministration of nitroprusside sodium. Maternal blood pressure and blood and urine chemistry studies were unaffected by NG-nitro-L-arginine methyl ester.

CONCLUSION

Chronic reductions of nitric oxide production in the last third of pregnancy result in significant intrauterine growth retardation and hemorrhagic disruptions of hind limbs. Maternal complications were minimal and did not mimic preeclampsia.